Combined transparent/non-transparent cache

ABSTRACT

In one embodiment, a memory that is delineated into transparent and non-transparent portions. The transparent portion may be controlled by a control unit coupled to the memory, along with a corresponding tag memory. The non-transparent portion may be software controlled by directly accessing the non-transparent portion via an input address. In an embodiment, the memory may include a decoder configured to decode the address and select a location in either the transparent or non-transparent portion. Each request may include a non-transparent attribute identifying the request as either transparent or non-transparent. In an embodiment, the size of the transparent portion may be programmable. Based on the non-transparent attribute indicating transparent, the decoder may selectively mask bits of the address based on the size to ensure that the decoder only selects a location in the transparent portion.

This application is a continuation of U.S. patent application Ser. No.13/545,526 filed Jul. 10, 2012 and now U.S. Pat. No. 8,566,526, which isa continuation of U.S. patent application Ser. No. 12/500,747, filedJul. 10, 2009 and now U.S. Pat. No. 8,244,981. The above applicationsare incorporated herein by reference in their entireties.

BACKGROUND

1. Field of the Invention

This invention is related to the field of integrated circuits, and moreparticularly to integrated circuits with on-chip memories.

2. Description of the Related Art

Various types of integrated circuits include on-chip memories. Forexample, integrated circuits can include caches. Integrated circuitsthat include processors often include caches to provide low latencyaccess to a subset of the data that is also stored in off-chip memories.Generally, caches are hardware-managed memories that store the mostrecently used data, and the cache management hardware writes copies ofdata accessed by the processors (or other memory-reading devices in theintegrated circuit) to the cache. Data that has been modified can bereplaced in the cache by newer data, and the cache management hardwarecan write the modified data back to the main memory. In some cases, theprocessors can include prefetch instructions and other cache hints toinfluence the operation of the cache management hardware and can markmemory non-cacheable to prevent caching, but generally software cannotcontrol the cache management hardware.

Another type of on-chip memory is embedded memory or “local memory”.Such memory is under software control (i.e. software reads and writesthe memory and thus directly controls which data is stored in theembedded memory). The embedded memory can have lower latency thatexternal memory, and if the data stored in the embedded memory isaccessed frequently, power savings can be achieved as compared toaccessing external memory.

SUMMARY

In one embodiment, an integrated circuit includes a memory that isdelineated into transparent and non-transparent portions. Thetransparent portion may be controlled by a control unit coupled to thememory, along with a corresponding tag memory. The non-transparentportion may be software controlled by directly accessing thenon-transparent portion via an input address, without a tag match. In anembodiment, the memory may include a decoder configured to decode theaddress and select a location in either the transparent ornon-transparent portion. Each request may include a non-transparentattribute identifying the request as either transparent ornon-transparent. In an embodiment, the size of the transparent portionmay be programmable. Based on the non-transparent attribute indicatingtransparent, the decoder may selectively mask bits of the address basedon the size to ensure that the decoder only selects a location in thetransparent portion. In an embodiment, software may assign an addressrange to the non-transparent portion that decodes to select onlylocations in the non-transparent portion of the memory.

In some embodiments, providing a programmable size of the transparentportion may permit cache sizing based on the workload being experiencedby the integrated circuit. The remaining portion of the memory may beused as non-transparent memory, and thus the entirety of the memory maybe useable for any desired cache size.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description makes reference to the accompanyingdrawings, which are now briefly described.

FIG. 1 is a block diagram of one embodiment of a system including one ormore combined transparent/non-transparent cache.

FIG. 2 is a block diagram of one embodiment of a combinedtransparent/non-transparent cache.

FIG. 3 is a block diagram of one embodiment of a page table entryincluding a non-transparent attribute.

FIG. 4 is a block diagram of a programmable register defining anon-transparent address range.

FIG. 5 is a flowchart illustrating operation of one embodiment of acombined transparent/non-transparent memory in response to a memoryrequest.

FIG. 6 is a flowchart illustrating operation of one embodiment of codethat may make use of the non-transparent portion of the combinedtransparent/non-transparent memory.

FIG. 7 is a block diagram illustrating one embodiment of a memoryaddress space.

FIG. 8 is a flowchart illustrating operation of one embodiment of ablock-based non-transparent cache in response to a block request.

FIG. 9 is a block diagram illustrating various states of each block inone embodiment of the block-based non-transparent cache and transitionsbetween the states.

FIG. 10 is a flowchart illustrating operation of one embodiment of codethat may request a non-transparent memory block and process data in theblock.

FIG. 11 is a block diagram of one embodiment of a system.

FIG. 12 is a block diagram of one embodiment of a computer accessiblestorage medium.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims. The headings used herein are for organizational purposes onlyand are not meant to be used to limit the scope of the description. Asused throughout this application, the word “may” is used in a permissivesense (i.e., meaning having the potential to), rather than the mandatorysense (i.e., meaning must). Similarly, the words “include”, “including”,and “includes” mean including, but not limited to.

Various units, circuits, or other components may be described as“configured to” perform a task or tasks. In such contexts, “configuredto” is a broad recitation of structure generally meaning “havingcircuitry that” performs the task or tasks during operation. As such,the unit/circuit/component can be configured to perform the task evenwhen the unit/circuit/component is not currently on. In general, thecircuitry that forms the structure corresponding to “configured to” mayinclude hardware circuits. Similarly, various units/circuits/componentsmay be described as performing a task or tasks, for convenience in thedescription. Such descriptions should be interpreted as including thephrase “configured to.” Reciting a unit/circuit/component that isconfigured to perform one or more tasks is expressly intended not toinvoke 35 U.S.C. §112, paragraph six interpretation for thatunit/circuit/component.

DETAILED DESCRIPTION OF EMBODIMENTS

Combined Transparent/Non-Transparent Cache Memory

In an embodiment, an integrated circuit includes an internal data memoryand an associated tag memory that is configured to store a set of tagscorresponding to at least a subset of the locations in the data memory.The portion of the data memory that is covered by the tags may be usedas a transparent cache memory. A transparent memory may generally bemanaged by hardware and thus software may not directly read/write thetransparent memory. If the data addressed by a software read/write (e.g.a load or store instruction) is stored in the transparent memory, thehardware will supply the data from the transparent memory (for a read)or update the data in the transparent memory (for a write). While thelatency for completing the memory operation may be reduced, the softwaremay not have any other indication that the memory operation wascompleted in the transparent memory (as opposed to completion in anexternal main memory system). Viewed in another way, the transparentmemory may not be separately mapped to memory addresses in the memoryaddress space, but rather may store copies of data from external memory,where the external memory locations are mapped to the associated memoryaddresses. The transparent memory may temporarily be mapped to theaddresses of the data stored in the transparent memory (e.g. via the tagmemory), but a corresponding main memory location is always mapped toeach address as well. The hardware will also ensure coherency of thedata (if coherency is implemented) and update the corresponding mainmemory copy if the data is modified in the transparent memory and isremoved from the transparent memory by hardware.

The remainder of the data memory that is not covered by the tags may beused as a non-transparent memory. Software may map the non-transparentmemory to a portion of the memory address space. For example, there maybe no main memory locations associated with the portion of the memoryaddress space that is mapped to the non-transparent memory.Alternatively, if there are main memory locations associated with theportion of the memory address space that is mapped to thenon-transparent memory, the locations will not be accessed by requestsgenerated by requestors that are coupled to the non-transparent memory.Accordingly, software may directly read/write the non-transparent memoryusing load/store instructions addressed to the addresses in the portionof the memory address space that is mapped to the non-transparentmemory. In an embodiment, software may manage the contents of thenon-transparent memory. For example, software may initialize thecontents of the non-transparent memory with store instructions or byprogramming a direct memory access (DMA) unit to transfer data into thenon-transparent memory from another source. Software may similarly readthe contents and write them to some other location (or use a DMAtransfer) to move data out of the non-transparent memory. In anotherembodiment described in more detail below for the block-basednon-transparent memory, the non-transparent memory may have associatedhardware to automatically move the data into or out of thenon-transparent memory responsive to block requests from software.

In an embodiment, the size of the transparent memory may beprogrammable. Thus, the amount of the data memory that is allocated tothe transparent memory may be changed to optimize the transparent memoryfor the workload being experienced in the integrated circuit. Forexample, for a given size of the data set operated upon by the workload(its “memory footprint”) and for a given pattern of accesses to the dataduring operation, increasing the transparent memory above a certain sizedoes not generally lead to significant increases in performance. The hitrate at the certain size for accesses to the data set may reach a highenough percentage that further size increases only increase the hit rateby small amounts. Accordingly, by programming the size of thetransparent portion, the amount of data memory dedicated to transparentmemory may be optimized and the remainder of the memory may be used asnon-transparent memory.

In an embodiment, the tag memory may have capacity for tags to coveronly a portion of the data memory, even if the transparent memory isprogrammed to its maximum size. Tag memory may frequently occupy morespace per bit of storage than a data memory, and thus limiting the tagsto a portion of the data memory may limit the size of the overalltransparent/non-transparent memory. Thus, efficient use of semiconductorarea may be achieved, in some embodiments. Alternatively, the tag memorymay have the capacity for tags for each cache block of the data memory.Such embodiments may permit the entire data memory to be allocated totransparent memory. In such embodiments, the tag memory corresponding tothe non-transparent portion of the data memory may be used to storestate information for the corresponding block, the memory address towhich the corresponding block is mapped, etc. In yet anotheralternative, a separate tag table may be implemented to store addressand state information corresponding to blocks in the non-transparentmemory, as desired.

The data memory, while having portions allocated to transparent andnon-transparent memory, may still be a single memory array, with asingle decoder that decodes addresses to access the array. Based on theallocation to transparent and non-transparent, the decoder may modifyits decode of the addresses to ensure that transparent accesses aredecoded into the transparent portion. The non-transparent portion may bemanaged by software such that a non-transparent access uses an addressthat naturally decodes to the non-transparent portion. Alternatively,the decoder may be configured to decode the addresses of non-transparentaccesses into the non-transparent portion. Using a single memory mayalso be efficient in terms of space, while providing the desiredtransparent and non-transparent functionality.

In some embodiments, there may be certain types of workload thatsoftware may be better at handling in the non-transparent memory thanthe hardware may handle in the transparent memory. For example, thehardware may retain recently accessed data in the transparent memory.Processing of a data set that does not involve much re-access ofpreviously accessed data may not benefit from transparent memory, butsoftware may be able to manage non-transparent on-chip memoryefficiently to provide lower latency access to the data (on average).Even for data that is frequently re-accessed, non-transparent memory maysometimes be more efficient and/or provide better performance thantransparent memory. For example, flushing an address range from atransparent cache without affecting/interfering with other cacheoperation may be challenging, while an address range may be flushed viahardware in the non-transparent memory, in one embodiment. In anotherexample, the replacement policy implemented in the cache may not suit aparticular workload, and permitting software to control allocation andeviction from the non-transparent memory may match the storage of datain the non-transparent memory and the workload. Other access patternsthat involve greater re-access of the data may benefit from thetransparent memory.

Turning now to FIG. 1, a block diagram of one embodiment of a systemincluding one or more combined transparent/non-transparent on-chipmemories is shown. In the embodiment of FIG. 1, the system includes oneor more graphics processing units (GPUs) 10A-10N, corresponding level 2(L2) caches 12A-12N, and a multi-core management block (MCMB) 14A. TheMCMB 14A includes a shared cache memory 16A, part of which may be alevel 3 (L3) transparent cache memory and part of which may be anon-transparent memory. The MCMB 14A also includes a control unit 18Acoupled to the shared memory 16A. The MCMB 14A is coupled to the L2caches 12A-12N, which are coupled to respective GPUs 10A-10N. The MCMB14A is also coupled to a main memory system 20. The system furtherincludes one or more central processing units (CPUs) 22A-22M,corresponding level 2 (L2) caches 24A-24M, and a multi-core managementblock (MCMB) 14B. The MCMB 14B includes a shared cache memory 16B, partof which may be a level 3 (L3) transparent cache memory and part ofwhich may be a non-transparent memory. The MCMB 14B also includes acontrol unit 18B coupled to the shared memory 16B. The MCMB 14B iscoupled to the L2 caches 24A-24M, which are coupled to respective CPUs22A-22M. The MCMB 14B is also coupled to a main memory system 20.Components referred to herein by a reference numeral followed by aletter may be similar (but not necessarily identical) to othercomponents having the same reference numeral and a different letter. Thecomponents having the same reference numeral and different letters maybe collectively referred to by the reference numeral alone (e.g. theGPUs 10A-10N may be referred to collectively as GPUs 10).

In general, the shared cache memories 16 may each be coupled to receivememory requests from one or more request sources. For example, in FIG.1, the GPUs 10 may be request sources for the memory 16A and the CPUs 22may be request sources for the memory 16B. As illustrated in FIG. 1, thememory requests may pass through other components (e.g. the L2 caches 12and 24 in FIG. 1, respectively) to reach the shared memory, and if thememory requests hit in the L2 caches 12 or 24, they may not reach theshared memory. The GPUs 10 and CPUs 22 may be configured to generatememory requests in response to executing load/store instructions, inresponse to instruction fetches, and in response to ancillary supportoperations such as address translations. While processors are used asrequest sources in this embodiment, any circuitry that may generatememory requests may be used.

The memory requests may include a non-transparent attribute identifyingthe request as either non-transparent or transparent. For example, thenon-transparent attribute may be a bit indicating non-transparent whenset and transparent when clear. The opposite meanings of the set andclear states of the bit may be used in other embodiments, and otherattribute encodings may be used.

If a memory request is indicated as non-transparent, the cache memory 16may be configured to decode the address into the non-transparent portionof the memory. The data stored at the identified location may beprovided in response to the memory request (if the request is a read) orupdated in response to the memory request (if the request is a write).That is, the address of the memory request may directly address thememory, without performing any tag comparison or other qualification ofthe address. Transparent memory requests, on the other hand, may bedecoded to address the transparent portion of the memory. Data from thelocation or locations may be provided/updated only if a tag match isdetected and the cache block is valid in the cache. Detecting a validtag match may be referred to as a cache hit (the addressed data isstored in the cache). Not detecting a valid tag match may be referred toas a cache miss (the addressed data is not stored in the cache). Thecontrol unit 18 may be configured to initiate a cache fill to copy theaddressed data into the cache in response to a cache miss. A location inthe cache may be selected to store the missing cache block, and if avalid, modified (dirty) cache block is stored in the selected location,the control unit 18 may be configured to write the modified cache blockback to the main memory system 20. On the other hand, there is noconcept of a miss in the non-transparent portion of the memory and thusthere may be no cache fill initiated for the non-transparent portions bythe control unit 18.

The non-transparent attribute may be determined in a variety offashions. In one embodiment, the non-transparent attribute may beincluded in the page table entry used to translate a virtual address toa physical address that is used to access memory. For example, the pagetables 26 stored in the main memory system 20 may include page tableentries that include the non-transparent attribute for each pagetranslated by the page tables. Software that controls the virtualaddress translations may assign the non-transparent attribute to eachpage. The software may be executed on the CPUs 22A-22M, the GPUs10A-10N, or both. In an embodiment, software may assign anon-transparent address range within the memory address space used toaccess the main memory system 20, and the non-transparent address rangemay be mapped to the non-transparent portion of the memory 16.Translations to pages in the non-transparent address range may have thenon-transparent attribute indicating non-transparent, and other pagesmay have the non-transparent attribute indicating transparent. Otherembodiments may use other mechanisms to determine the non-transparentattribute. For example, the non-transparent address range may beprogrammed into one or registers accessible in the memory request path(e.g. in a memory management unit in the GPUs 10 or 22; in an addressgeneration unit in the processors 10 or 22; in the memory 16 or itscontrol unit 18; etc.). Other embodiments may specify thenon-transparent attribute as an operand of the instruction, via aspecific instruction encoding, etc.

The GPUs 10A-10N may implement an instruction set architecture that isoptimized for graphics manipulations (e.g. rendering images into a framebuffer, pixel manipulations, etc.). The GPUs 10A-10N may implement anymicroarchitecture, including scalar, superscalar, pipelined,superpipelined, out of order, in order, speculative, non-speculative,etc., or combinations thereof. The GPUs 10A-10N may include circuitry,and optionally may implement microcoding techniques. Similarly, the CPUs22A-22M may implement an general purpose instruction set architectureand may implement any microarchitecture, including any of the abovepossibilities. The GPUs and CPUs may be examples or processors, whichmay be any circuitry configured to execute instructions. A processor maybe a discrete integrated circuit, a core integrated onto an integratedcircuit, etc. For example, in FIG. 1, the GPUs 10, L2 caches 12, andMCMB 14A may be integrated onto a graphics chip and the CPUs 22, L2caches 24, and MCMB 14B may be integrated onto a multi-core CPU chip. Inanother embodiment, the GPUs 10, CPUs 22, L2 caches 12 and 24, and MCMBs14 may be integrated into an integrated circuit. In some embodiments,the integrated circuit may also include other components integrated withthe GPUs/CPUs and related circuitry.

The GPUs 10 and CPUs 22 may include L1 caches (not shown), and thus thecaches 12 and 24 are L2 caches in this embodiment. The L2 caches mayhave any size and configuration (e.g. set associative, direct mapped,etc.). The L2 caches may also implement any cache block size (e.g. 32bytes or 64 bytes, or larger or smaller). The cache block size may bethe unit of allocation and deallocation in the cache.

In addition to including the combined cache memories 16, the MCMBs 14may generally provide interconnect between the corresponding processorsthe main memory system 20. If cache coherency is implemented, the MCMBs14 may be responsible for issuing probes (e.g. a request from oneprocessor may result in a probe to the other processors to obtain anymodified data in the other processor's L1 or L2 caches, invalidatingcached copies for any update request, etc.). The MCMBs may communicatewith each other and/or with a memory controller in the main memorysystem 20. In one embodiment, the memory controller may be implementedon chip with the MCMBs 14, and/or may be part of one of the MCMBs 14.

The main memory system 20 may include any type of memory. For example,the memory may comprise dynamic random access memory (DRAM), synchronousDRAM (SDRAM), double data rate (DDR, DDR2, DDR3, etc.) SDRAM (includingmobile versions of the SDRAMs such as mDDR3), RAMBUS DRAM (RDRAM),static RAM (SRAM), etc.

Other embodiments of the system of FIG. 1 may include only one sharedcache memory (e.g. either the memory 16A/control unit 18A or the memory16B/control unit 18B). Still other embodiments may have a shared memoryaccessible to both the CPUs 22A-22M and the GPUs 10A-10N, which may besimilar to the memory 16/control unit 18 as described above.

It is noted that the number of each component in various embodiments mayvary. For example, one or more GPUs 10A-10N may be provided, and one ormore CPUs 22A-22M is provided. In other embodiments, there may be noGPUs and/or no CPUs. As indicated by 10N and 22M, the number of oneprocessor may differ from the number of the other processor. L2 caches12 and 24 are shown in FIG. 1 to be associated with respectiveprocessors, but may be shared by subsets of the processors in otherembodiments. In still other embodiments, there may be no L2 caches.

Turning now to FIG. 2, a block diagram of one embodiment of the memory16A and control unit 18A is shown. The memory 16B and control unit 18Bmay be similar. In the illustrated embodiment, the memory 16A includesdecoders 30A-30B, a tag memory 32, a data memory 34, and a comparator36. The control unit 18A includes a delineation register 38. Thedecoders 30A-30B are coupled to receive an address of a memory request(Address in FIG. 2), the non-transparent attribute of the request (NT inFIG. 2), and other attributes of the request (Other Attributes in FIG.2). The control unit 18A is coupled to receive the non-transparentattribute and may also receive the other attributes, or some of theother attributes, in various embodiments. The comparator is coupled toreceive the non-transparent attribute, the other attributes (or at leastsome of the other attributes), and the address (or at least the portionof the address that is compared to the tags from the tag memory 32). Thedecoder 30A is coupled to the tag memory 32 and the control unit 18A,and the decoder 30B is coupled to the data memory 34 and the controlunit 18A. The data memory 34 is coupled to the control unit 18A, toprovide read data output to the L2 caches and main memory system, and toreceive write data from the L2 caches. The tag memory 32 is coupled tothe comparator 36, which is coupled to the data memory 34 and thecontrol unit 18A.

The decoders 30A-30B may be configured to decode the address of thememory request to select a memory location in the tag memory 32 and thedata memory 34 to be accessed, respectively. The location in the tagmemory 32 may store one or more tags to be compared to the tag portionof the address for a transparent memory request. The number of tagsstored in the location may depend on the configuration of thetransparent cache. For example, if the transparent cache is directmapped, one tag may be stored. If the transparent cache is N-way setassociative, N tags may be stored. The tags may be output by the tagmemory 32 to the comparator 36, along with various state such as a validbit. The comparator 36 may compare the tags to the tag portion of theaddress, and signal a hit or miss to the data memory 34 and the controlunit 18A. If the cache is N-way set associative, the comparator 36 mayalso identify the hitting way. The data memory 34 may output data fromthe hitting way (or may write the write data to the hitting way, for awrite). The tag portion of the address may be the portion of the addressthat excludes bits identifying the offset of the memory request within acache block and also exclude the index bits that are decoded by thedecoders 30A-30B to select locations.

The comparator 36 may receive the non-transparent attribute, and may beconfigured to inhibit signalling a hit for non-transparent memoryaccesses. Instead, the data memory 34 may access the identified locationin the non-transparent portion of the data memory 34 in response to anon-transparent memory request. Similarly, the other attributes of thememory request may also affect the comparison. For example, the otherattributes may include a non-cacheable attribute and/or a cache bypassattribute. If the memory access is non-cacheable or the cache isbypassed, the comparator 36 may inhibit asserting hit for the access aswell. The control unit 18A may interface with other circuitry in theMCMB 14A to initiate the main memory access for the memory request(to/from main memory system in FIG. 2). Alternatively, in someembodiments, tags may be maintained for the non-coherent portion andcomparators 36 may make the comparison.

The decoder 30B may similarly be configured to decode the address andselect a location to be accessed. As illustrated in FIG. 2 by thehorizontal dashed line 40, the data memory 34 may be divided into atransparent cache portion (above the dashed line 40 in FIG. 2) and anon-transparent memory portion (below the dashed line 40 in FIG. 2).Accordingly, non-transparent memory addresses may decode to locations inthe non-transparent portion and transparent memory addresses may decodeto locations in the transparent portion. In one embodiment, for example,the transparent portion of the data memory 34 may be mapped to thelowest numerical values of the index. In such an embodiment, the decoder30B may mask address bits of the index outside of the range that ismapped to the transparent portion for transparent memory requests,ensuring that any transparent memory request address decodes to thetransparent portion. That is, if the non-transparent attribute indicatestransparent, the decoder 30B may mask the more significant bits of theindex to zero, forcing the index to decode into the transparent portion.Non-transparent memory request addresses may not be masked, and may thusdecode into the non-transparent portion. In one embodiment, software maycontrol the allocation of the non-transparent memory address range sothat any address in the range has non-zero address bits in the portionof the index that is masked for transparent memory requests (and thusnon-transparent memory request addresses decode into the non-transparentportion without any specific logic in the decoder 30B other than normaladdress decoding).

For example, the data memory 24 may include 4096 addressable memorylocations, and thus the index for the data memory may include 12 addressbits. If 256 of the memory locations are allocated to the transparentcache, the least significant 8 address bits may be decoded for atransparent memory request (the most significant 4 bits may be masked)and the non-transparent memory address range may include non-zero bitsin the 4 most significant bits of the index. Other embodiments may maptransparent and non-transparent addresses to data memory locations inother ways.

In one embodiment, the size of the transparent cache is programmable inthe delineation register 38. In such an embodiment, the control unit 18Amay provide masking controls to the decoders 30A-30B to mask additionaladdress bits for the programmed size of the cache. Continuing with theabove example, if the transparent cache is programmed to ½ size (128memory locations), then one additional most significant bit (i.e. the5th most significant bit) is masked. The decoder 30A may also mask theindex in this case, to reduce the tag access to match the programmedsize of the cache. The programmed delineation may also increase the sizeof the non-transparent memory, since the portion not being used for thetransparent cache memory may be used for the non-transparent memory.

Other embodiments may use different decoder 30A-30B configurations. Forexample, the non-transparent memory range may be freely allocated to anybase address, and the decoder 30B may decode the range into thenon-transparent portion of the data memory by decoding the base addressto the first location in the non-transparent portion, decoding the baseaddress plus the size of the first location to the second location, etc.Another embodiment may also use different schemes of mapping thetransparent cache and non-transparent memory to locations in the datamemory 34.

In some embodiments, the data memory 34 may be banked. For example, a 64byte cache block may be stored across eight 8 byte banks in the datamemory 34. In such an embodiment, the non-transparent memory may have afiner granularity than the cache block. For example, a non-transparentmemory access may be bank sized, such as 8 bytes in the above example.

As illustrated in FIG. 2, the data memory 34 may be a single memoryarray managed by a single decoder. That is, the decoder may decodeaddresses to generate a set of word lines, each word line selecting adifferent location in the memory array forming the data memory 34. Someof the locations may be cache block storage locations for thetransparent cache memory, and other locations may be the non-transparentmemory locations.

In another embodiment, the non-transparent portion of the memory mayalso be tagged. Such embodiments may permit multiple address ranges tobe assigned as non-transparent addresses. However, in such embodiments,the tag memory for the non-transparent memory may be readable andwriteable by software so that software may manage the contents of thenon-transparent portion.

It is noted that, while FIG. 2 illustrates one address input to thememory 16A, other embodiments may support more than one address inparallel in a multiported configuration. Decoders similar to 30A-30B maybe included for each port.

Turning now to FIG. 3, a block diagram of one embodiment of a page tableentry 50 that may be stored in the page table 26 is shown. The pagetable entry 50 may be used as part of any address translation mechanism.In some embodiments, multiple page table accesses in a hierarchicalfashion may be used to map a virtual address to a physical address. Insuch embodiments, the virtual address tag (VA Tag) may not be needed. Inother embodiments, a portion of the virtual address that is not used tolook up the page table entry may be matched against the virtual addresstag field of the entry 50. A physical page number (PPN) field may storethe page portion of the physical address (which may be concatenated withthe offset portion of the virtual address to form the physical address).Any page size may be supported (e.g. 4 kilobytes, 8 kilobytes, or largersuch as 1-4 Megabytes, or even larger). In some embodiments, more thanone page size maybe supported. The page table entry 50 also includes thenon-transparent attribute (NT), and may include other attributes (OtherAttributes field). The other attributes may include, for example,cacheability, writethrough or writeback, privilege level requirements, avalid bit, read/write permissions, etc.

Accordingly, using page table entries such as entry 50, software mayallocate a range of physical addresses as non-transparent using the NTattribute. Other pages outside of the range may have the NT attributeindicating transparent.

FIG. 4 is a block diagram of an embodiment of a register 52 that may beprogrammed with a non-transparent address range. The address range maybe expressed in any fashion. For example, as shown in FIG. 4, theaddress range may be represented as a base address and a limit. Therange may also be represented as a base address and a size, or may berepresented in any other fashion that defines the range. A register suchas register 52 may be used at any point in an address generation andtranslation (if applicable) for a memory request to determine thenon-transparent attribute of the memory request. For example, a memorymanagement unit (MMU) in a processor may include the register 52, andtranslated addresses may be compared to the base and limit fields todetermine if the address is in the non-transparent address range oroutside of the non-transparent address range. More than one addressrange may be defined, in some embodiments, by including more than oneregister 52.

Turning next to FIG. 5, a flowchart is shown illustrating operation ofone embodiment of the memory 16A/control unit 18A in response to amemory request presented to the memory 16A/control unit 18A. While theblocks are shown in a particular order for ease of understanding, otherorders may be used. Blocks may also be performed in parallel bycombinatorial logic within the control unit 18A. Blocks, combinations ofblocks, and/or the flowchart as a whole may be pipelined over multipleclock cycles.

If the non-transparent attribute of the request indicates transparent(decision block 54, “no” leg), the decoders 30A-30B may be configured tomask the address of the memory request to ensure that the addressdecodes into the transparent portion of the data memory 34 (block 56).If the non-transparent attribute indicates non-transparent (decisionblock 54, “yes” leg), no masking may occur. In either case, the decoders30A-30B may be configured to decode the address (block 58) and selectmemory locations in the tag memory 32 and the data memory 34. If thememory request is transparent and is a miss in the transparent portionof the memory 16A (decision block 60, “yes” leg), the control unit 18Amay be configured to generate a cache fill to obtain the missing cacheblock (block 62). The control unit 18A may be configured to select ablock to evict from the cache, and if the evicted block is modified, thecontrol unit 18A may be configured to write the cache block back to themain memory system 20. If the memory request is a hit or isnon-transparent (decision block 60, “no” leg), the memory location inthe data memory 34 may be accessed.

If the memory request is a read (decision block 64, “yes” leg), thememory 16A may output data from the accessed location in the data memory34 (block 66). Otherwise, the memory request is a write (decision block64, “no” leg) and the memory 16A may update the accessed location withthe write data (block 68).

Turning next to FIG. 6, a flowchart is shown illustrating operation ofone embodiment of control code that may be executed by one or both ofthe CPUs 22 or the GPUs 10 to control the memory 16A. Similar operationmay be performed for the memory 16B. The control code may compriseinstructions which, when executed, may cause the system to implement theoperation illustrated in FIG. 6. While the blocks are shown in aparticular order in FIG. 6, other orders may be used.

The code may determine the desired size of the transparent cache portionof the memory 16A (block 70). The desired size may be the maximum size,or less than the maximum size. Various factors may affect thedetermination. For example, the workload that is to be executed mayaffect the size. If the workload will benefit from a larger cache size(e.g. because the expected hit rate will go up), a larger cache size maybe selected. If the workload will not benefit from a large cache (e.g.there is not much reuse of the data as the workload executes), then asmaller cache size may be used. The code may program the size into thecontrol unit 18A (e.g. in the delineation register 38—block 72).

Dependent on the selected transparent cache size and size of the datamemory 34, the code may determine a base address and size of thenon-transparent memory (block 74). For example, if a cache size of 1Megabyte is selected, the base address of the non-transparent memoryrange may be on a 1 Megabyte boundary. The size of the non-transparentmemory may be the data memory size less the size of the transparentcache (e.g. if the data memory is 8 Megabytes and the cache size is 1Megabyte, the non-transparent memory is 7 Megabytes beginning at a 1Megabyte boundary). The code may program the configuration registers orpage table entries to identify the non-transparent memory, depending onthe embodiment (block 76). The code may manage the contents of thenon-transparent memory (block 78). For example, the code may initializethe non-transparent memory to a known value, may move data between thenon-transparent memory from other memory locations or a peripheral, etc.In some embodiments, the movement of data between the non-transparentmemory and other memory locations or a peripheral may be handled inhardware circuitry.

Turning next to FIG. 7, a block diagram of one embodiment of a memoryaddress space 80 is shown. The memory address space may comprise a setof numerical values which are mapped to memory in the system. That is,each numerical value within the memory address space 80 uniquelyidentifies a specific storage location in the memory of the system. Insome cases, a portion of the memory address space 80 may be mapped toperipheral devices (memory-mapped input/output (I/O)), but the remainderof the memory address space is mapped to memory locations. Addresses ina different address space (e.g. an I/O address space or a configurationaddress space) may not be comparable to the addresses in the memoryaddress space.

As illustrated in FIG. 7, a portion of the memory address space 80 ismapped to the non-transparent portion of the memory 16A (referencenumeral 82). The location of the non-transparent range 82 within thememory address space may be determined by software, and may be subjectto beginning at a specific boundary based on the size of the transparentcache portion in some embodiments. Other memory addresses in the memoryaddress space (e.g. addresses below the non-transparent range 82,illustrated at reference number 84, and addresses about thenon-transparent range 82, illustrated at reference numeral 86) may bemapped to the main memory system 20. Addresses in the ranges 84 and 86may be eligible for caching in the transparent cache portion of thememory 16A, if indicated as cacheable in the page tables or via othermechanisms for determining cacheability such a memory type regionregisters that may be supported in the CPUs 22 or the GPUs 10.

In FIG. 7, the numerical addresses within the memory address space 80are illustrated on the left side of the space. Accordingly, the lowermain memory address range 84 begins with address 0 and extends toaddress N. The address N may be any address that is a boundary of thesize of the transparent cache portion of the memory 16A. Thenon-transparent address range thus begins with the address N+1 andextends to the address N+M (where M is the size of the non-transparentrange). The upper main memory address range begins with address N+M+1and extends to N+M+Q. Accordingly, any address between 0 and N+M+Q mapsto a specific memory location in either the main memory system 20 or thenon-transparent portion of the memory 16A. The address N+M+Q is eitherthe maximum possible address in the system, or addresses larger thanN+M+Q may cause a fault in the instruction that generates the address.

Block-Based Non-Transparent Memory

The non-transparent portion of the memory 16A may be sufficient to storea non-transparent data set for the GPUs 10 (that is, the data set thatsoftware desires to map to the non-transparent memory in the memory16A). Similarly, the non-transparent memory 16B may be sufficient tostore a non-transparent data set of the CPUs 22. In other embodiments,the desired non-transparent data set may exceed the size of thenon-transparent memory. For such embodiments, software may need totransfer data into and out of the non-transparent memory fairlyfrequently. An embodiment of the control unit 18A (or 18B) iscontemplated that provides assistance in performing the data movements.

Some embodiments may be implemented in the context of the embodiment ofFIG. 1, in which a portion of the memory 16A is also allocated totransparent cache memory. Other embodiments, however, may be implementedin a non-transparent memory that is not shared with a transparent cachememory in the same memory array (but that is still on-chip with therequest sources that generate the memory requests to the non-transparentmemory).

The non-transparent memory address range may be divided into multiplenon-transparent memory blocks. A non-transparent memory block is acontiguous (in the memory address space) block of non-transparent memorythat may be requested by a request source (e.g. a GPU 10A-10N or CPU22A-22M). The non-transparent memory block may be any desired size, andneed not be related to the size of a cache block. For example, anon-transparent memory block may be the size of a page in the addresstranslation mechanism implemented within the system, or may be aninteger multiple of a page in size. Other embodiments may use a sizesmaller than a page size as well, if desired.

For simplicity in the remainder of the description, the GPUs 10A-10Nwill be used as an example of request sources and the non-transparentportion of the memory 16A and control unit 18A as an example of thenon-transparent memory. However, other embodiments may have otherrequest sources and non-transparent memories, including the CPUs 22A-22Mand the memory 16B/control unit 16B (or a single shared memory for CPUand GPU requestor sources).

In addition to performing various reads and writes to non-transparentmemory, the GPUs 10 may be configured to transmit a block request to thecontrol unit 18A. The block request may identify a usage model for theblock, and may also identify a main memory address (that is, an addressthat is not mapped to the non-transparent memory). The control unit 18Amay be configured to allocate one of the non-transparent memory blocksin response to the block request, and may be configured to return apointer to the block (e.g. a base address of the non-transparent memoryblock). Based on the usage model, the control unit 18A may automaticallymove data between the main memory and the allocated block. For example,the control unit 18A may automatically fill data from the main memoryinto the allocated block, automatically flush data from the allocatedblock to main memory after the GPU 10 indicates that it is finished withthe allocated block, or both.

The block request may be formulated in any fashion. For example, theblock request may be a store instruction to a defined address that isassigned as the block request address followed by a load instruction tothe same address. The data stored by the store instruction may be themain memory address that is the source/destination of data for theallocated block and may also include an indication of the usage modeldescribed below. The block address (the base address of the assignedblock) may be returned as the result of the load instruction, and thusthe software executing on the GPU may have access to the block using theblock address. Alternatively, a specific instruction may be defined inthe GPU's instruction set architecture to transmit the block request.Any mechanism for transmitting the block request and receiving the blockaddress may be used.

In one embodiment, there may be three possible usage models for anon-transparent memory block: static read, static write, and staticread/write. Other embodiments may define any set of two or more usagemodels, as desired. A static read usage model includes automaticallyflushing the data from the allocated block to main memory when the blockrequestor is finished with the block. Thus, a static read block may be ablock in which the block requestor intends to write each byte of theblock (or the contents on unwritten bytes of the block are “don'tcare”). A static write usage model includes automatically filling thedata from the main memory. A static read/write model includes bothautomatically filling and automatically flushing.

By providing the block allocation and automatic data movement, thecontrol unit 18A may relieve the software executing on the GPUs from thedata movement tasks. In some cases, performance may be improved sincethe software need not move the data using load/store instructions.

Turning now to FIG. 8, a flowchart is shown illustrating operation ofone embodiment of the control unit 18A in response to a block requestfor a non-transparent memory block. While the blocks are shown in aparticular order for ease of understanding, other orders may be used.Blocks may be performed in parallel in combinatorial logic in thecontrol unit 18A. Blocks, combinations of blocks, and/or the flowchartas a whole may be pipelined over multiple clock cycles.

In one embodiment, a block request for a given requestor may also be anindication that the requestor is finished with a previously allocatednon-transparent memory block. If a previous non-transparent memory blockwas allocated to the requestor (decision block 90, “yes” leg), and theprevious non-transparent memory block was a static read type (decisionblock 92, “yes” leg), the control unit 18A may be configured to startthe flush of the previous non-transparent memory block (block 94). Thecontrol unit 18A may be configured to change the state of thenon-transparent memory block to flushing. The previous non-transparentmemory block may have the static read type if the usage model wasindicated as static read or static read/write when the previousnon-transparent memory block was requested. If the previousnon-transparent memory block is not the static read type, the controlunit 18A may be configured to change the state of the non-transparentmemory block to available (block 96).

In other embodiments, an explicit block complete command may besupported, and the requestor may transmit the block complete command tothe control unit 18A. The control unit 18A may be configured to performthe same processing illustrated via blocks 92, 94, and 96 in response tothe block complete command.

The control unit 18A may be configured to determine if a non-transparentmemory block is available for allocation in response to the blockrequest (decision block 98). If not (decision block 98, “no” leg), thecontrol unit 18A may wait for a non-transparent memory block to becomeavailable (block 100). Alternatively, the control unit 18A may beconfigured to return a “no block available” response to the blockrequest if a non-transparent memory block is not available forallocation. The “no block available” response may be a zero blockaddress, or may be indicated in the least significant bits of the blockaddress which would be an offset within the block. If a non-transparentmemory block is available or becomes available after the wait, thecontrol unit 18A may be configured to allocate the non-transparentmemory block (block 102). The control unit 18A may also be configured torecord the corresponding main memory address supplied in the blockrequest, and may associate the corresponding main memory address withthe allocated block (block 104).

If the block request includes a static write type (e.g. the usage modelis static write or static read/write—decision block 106, “yes” leg), thecontrol unit 18A may be configured to initiate a fill of the allocatedblock from the corresponding main memory address (block 108). Thecontrol unit 18A may be configured to change the state of the allocatedblock to filling (block 110). If the block request does not include astatic write type (decision block 106, “no” leg), the control unit 18Amay be configured to change the state of the block to active (block112). In either case, the control unit 18A may be configured to returnthe block address of the allocated block to the requestor (block 114).

Turning now to FIG. 9, a block diagram illustrating a state machine thatmay be implemented by one embodiment of the control unit 18A for anon-transparent memory block is shown. The state machine of FIG. 9 maybe implemented for each non-transparent memory block in parallel. Thestate machine includes an available state 130, a filling state 122, anactive state 124, and a flushing state 126. Some of the transitionsillustrated in FIG. 9 were also mentioned above with regard to FIG. 8.

If the non-transparent memory block is in the available state 120, it isavailable to be allocated in response to a block request from arequestor. If the control unit 18A allocates the block for a blockrequest having a static write type (e.g. static write or staticread/write usage models), the control unit 18A may be configured tochange the state of the block from the available state 120 to thefilling state 122. In one embodiment, the control unit 18A may beconfigured to stall or reject read memory requests to the block whilethe block is in the filling state 122 (since data is still being writtento the block by the control unit 18A, moving data in from thecorresponding main memory address supplied by the requestor). In thefilling state 122, the control unit 18A may be configured to read thecorresponding main memory address (and addresses contiguous to thecorresponding main memory address) and to write the data to theallocated block. Once the fill is complete, the control unit 18A may beconfigured to change the state of the block from the filling state 122to the active state 124. On the other hand, if the control unit 18Aallocates the block in response to a block request that does not havethe static write type, the control unit 18A may change the state of theblock from the available state to the active state 124.

In the active state 124, the non-transparent memory block is allocatedto the requestor and the requestor may access the non-transparent memoryblock as desired. Once the requestor has completed processing of thedata in the non-transparent memory block, the requestor may indicatethat it is finished with the block (e.g. by requesting another block orusing the explicit block complete command as described above). If therequestor is done with the block and it is not a static read type (e.g.static read or static read/write usage model), the control unit 18A maychange the state of the block from the active state 124 to the availablestate 120. If the block is the static read type, the control unit 18Amay be configured to change the state of the block from the active state124 to the flushing state 126. In the flushing state 126, the controlunit 18A may be configured to write the data from the block to thecorresponding main memory address. When the flush is completed, thecontrol unit 18A may be configured to change the state of the block fromthe flushing state 126 to the available state 120.

Turning next FIG. 10, a flowchart is shown illustrating operation of oneembodiment of code executing on the GPUs 10 to process data using thenon-transparent memory blocks. The code may include instructions which,when executed, cause the system of FIG. 1 to implement the operationillustrated in FIG. 10. While the blocks are shown in a particular orderfor ease of understanding, other orders may be used.

The code may determine the block type and the main memory address of theblock to be processed (block 130). The block type may be based on theprocessing that the code is to do on the block. For example, if the codeis going to create new data to write to the block, the usage model maybe static read. If the code is going to read data from the block but notwrite it, the usage model may be static write. If the code is going toread data from the block and write the data, the usage model may bestatic read/write. The main memory address may be determined based onthe location of the data to be processed. For example, the GPUs 10 maybe processing tiles of an image in a frame buffer in the main memorysystem. The tiles may be subsections of the overall image. The GPU mayselect the next tile to be processed, and the main memory address may bethe address of the selected tile.

The code may request a non-transparent memory block (block 132) and mayuse the block address to perform processing in the non-transparentmemory block (block 134). Optionally, in embodiments that include anexplicit block complete command, the code may transmit the blockcomplete command (block 136). If there are additional blocks to beprocessed in the main memory (decision block 138, “yes” leg), the codemay return to block 130 to begin processing for the next block.

It is noted that, while specific functionality has been described hereinas being implemented in software or hardware, the functionalityimplemented in software and hardware may vary from embodiment toembodiment. For example, software may allocate the address region(s)that are defined to be non-transparent memory, and the hardware may beconfigured to move data into and out of the non-transparent memory.

It is noted that, in some embodiments, the communication between thehardware circuits and software code being executed may be in the form ofblock requests and examining the contents of the returned block. Stillfurther, the communication may be in the form of load/store commandswith various attributes to identify the particular communication.

System and Computer Accessible Storage Medium

Turning next to FIG. 11, a block diagram of one embodiment of a system150 is shown. The system 150 may be another embodiment of the systemillustrated in FIG. 1. In the illustrated embodiment, the system 150includes at least one instance of an integrated circuit 152 coupled toone or more peripherals 154 and an external memory 158. The integratedcircuit 152 may include the GPUs 10, the CPUs 22, the L2 caches 12 and24, the MCMBs 14, the memories 16, and the control units 18. Theexternal memory 158 may include the main memory system 20. A powersupply 156 is also provided which supplies the supply voltages to theintegrated circuit 152 as well as one or more supply voltages to thememory 158 and/or the peripherals 154. In some embodiments, more thanone instance of the integrated circuit 152 may be included (and morethan one external memory 158 may be included as well).

The peripherals 154 may include any desired circuitry, depending on thetype of system 150. For example, in one embodiment, the system 150 maybe a mobile device (e.g. personal digital assistant (PDA), smart phone,etc.) and the peripherals 154 may include devices for various types ofwireless communication, such as wifi, Bluetooth, cellular, globalpositioning system, etc. The peripherals 154 may also include additionalstorage, including RAM storage, solid state storage, or disk storage.The peripherals 154 may include user interface devices such as a displayscreen, including touch display screens or multitouch display screens,keyboard or other input devices, microphones, speakers, etc. In otherembodiments, the system 150 may be any type of computing system (e.g.desktop personal computer, laptop, workstation, net top etc.).

Turning now to FIG. 12, a block diagram of a computer accessible storagemedium 200 is shown. Generally speaking, a computer accessible storagemedium may include any storage media accessible by a computer during useto provide instructions and/or data to the computer. For example, acomputer accessible storage medium may include storage media such asmagnetic or optical media, e.g., disk (fixed or removable), tape,CD-ROM, or DVD-ROM, CD-R, CD-RW, DVD-R, DVD-RW. Storage media mayfurther include volatile or non-volatile memory media such as RAM (e.g.synchronous dynamic RAM (SDRAM), Rambus DRAM (RDRAM), static RAM (SRAM),etc.), ROM, Flash memory, non-volatile memory (e.g. Flash memory)accessible via a peripheral interface such as the Universal Serial Bus(USB) interface, a flash memory interface (FMI), a serial peripheralinterface (SPI), etc. Storage media may include microelectromechanicalsystems (MEMS), as well as storage media accessible via a communicationmedium such as a network and/or a wireless link. The computer accessiblestorage medium 200 in FIG. 12 may store control code 202, which mayinclude the code described above with regard to FIG. 6 and/or 10.Generally, the computer accessible storage medium 200 may store any setof instructions which, when executed, implement a portion or all of theoperation shown in FIGS. 6 and 10. A carrier medium may include computeraccessible storage media as well as transmission media such as wired orwireless transmission.

Numerous variations and modifications will become apparent to thoseskilled in the art once the above disclosure is fully appreciated. It isintended that the following claims be interpreted to embrace all suchvariations and modifications.

What is claimed is:
 1. A memory unit comprising: a tag memory configuredto store a plurality of tags corresponding to a plurality of cacheblocks; a data memory configured to store data including the pluralityof cache blocks, wherein the data memory is delineated into atransparent portion storing the plurality of cache blocks and anon-transparent portion that is directly mapped to a memory addressrange in a memory address space to which a main memory is also directlymapped for one or more other address ranges of the memory address space,wherein the transparent portion and the non-transparent portion arenon-overlapping; and a decoder coupled to the data memory, wherein thedecoder is configured to decode addresses of memory requests presentedto the memory unit to select corresponding entries in the data memory tobe accessed, and wherein the decoder is coupled to receive anon-transparent attribute for each memory request, and wherein thenon-transparent attribute indicates whether or not the memory request isa non-transparent memory request, and wherein the decoder is configuredto decode the address to access the transparent portion responsive tothe non-transparent attribute indicating transparent, and wherein thedecoder is configured to decode the address to access thenon-transparent portion responsive to the non-transparent attributeindicating non-transparent.
 2. The memory unit as recited in claim 1wherein the entry selected in the non-transparent portion is controlleddirectly by the address.
 3. The memory unit as recited in claim 2wherein the entry selected in the transparent portion is controlled bythe address and a hit detected from the tag memory for the address inthe entry.
 4. The memory unit as recited in claim 1 further comprising acontrol unit, wherein the delineation is programmable in the controlunit, and wherein the control unit is configured to provide anindication of the delineation to the decoder and to a second decodercoupled to the tag memory and configured to decode the address to selectan entry in the tag memory, and wherein the decoder and the seconddecoder are configured to decode the address for transparent requestsfurther responsive to the indication.
 5. The memory unit as recited inclaim 4 wherein the decoder and the second decoder are configured toselectively mask address bits in the decode responsive to theindication.
 6. An apparatus comprising: one or more request sourcesconfigured to generate memory requests; and a memory unit coupled to theone or more request sources, wherein the memory unit comprises a firstportion configured as a transparent cache tagged using a plurality oftags and a second portion configured as a non-transparent memory,wherein a non-transparent memory access is controlled directly by aninput address corresponding to the memory request, and wherein atransparent memory request is controlled by the input address and a tagmatch to one or more of the plurality of tags, wherein the first portionand the second portion are non-overlapping.
 7. The apparatus as recitedin claim 6 wherein each memory request includes an attribute indicatingwhether or not the corresponding memory request is non-transparent, andwherein the memory unit is configured to select an entry in thenon-transparent portion responsive to the attribute indicatingnon-transparent.
 8. The apparatus as recited in claim 7 wherein thememory unit is configured to select an entry in the transparent portionresponsive to the attribute indicating transparent and furtherresponsive to the tag match.
 9. The apparatus as recited in claim 6wherein the one or more request sources comprise one or more processors.10. The apparatus as recited in claim 9 wherein the one or moreprocessors comprise graphics processors.
 11. The apparatus as recited inclaim 6 wherein the second portion is directly mapped to one or morememory address ranges in a memory address space to which a main memoryis also directly mapped for one or more other address ranges of thememory address space.
 12. A method comprising: a memory unit receiving afirst memory access having a first address and a first attributeindicating non-transparent; the memory unit accessing a non-transparentportion of a memory within the memory unit responsive to the firstmemory access, wherein the non-transparent portion is directly mapped toat least a first address range within a memory address space; the memoryreceiving a second memory access having a second address and a secondattribute indicating transparent; and the memory unit accessing atransparent portion of the memory responsive to the second memoryaccess, wherein the transparent portion and the non-transparent portionare non-overlapping and non-zero in size.
 13. The method as recited inclaim 12 wherein the memory unit accessing the non-transparent portioncomprises: decoding the first address to identify a location in thenon-transparent portion; and accessing the non-transparent portionresponsive to the decoding and independent of any tag match.
 14. Themethod as recited in claim 12 wherein the transparent portion is taggedusing a plurality of cache tags that identify memory locations in a mainmemory that is mapped to other memory address ranges of the memoryaddress space than the first memory address range.
 15. The method asrecited in claim 14 wherein the memory unit accessing the transparentportion comprises: decoding the second address to identify a location inthe transparent portion; accessing a tag memory to select one or moretags corresponding to the location; performing a tag match between theone or more tags and the second address; and accessing the transparentportion responsive to the decoding of the second address and furtherresponsive to detecting a hit in the tag match.
 16. The method asrecited in claim 15 further comprising detecting a cache miss in the tagmatch and performing a cache fill in response to the cache miss.
 17. Themethod as recited in claim 16 further comprising writing the cache fillto the transparent portion.
 18. The method as recited in claim 12further comprising executing software instructions in a processorcoupled to the memory unit to manage a content of the non-transparentportion.
 19. The method as recited in claim 18 wherein the softwareinstructions copy data from a second memory address range of the memoryaddress space into the non-transparent portion, wherein the secondmemory address range is different from the first memory address range.20. The method as recited in claim 18 wherein the software instructionscopy data from the non-transparent portion to a second memory addressrange within the memory address space, wherein the second memory addressrange is different from the first memory address range.